def is_prime(num):
    if num < 2:
        return False
    for i in range(2, int(num ** 0.5) + 1):
        if num % i == 0:
            return False
    return True

def largest_prime(num):
    if len(str(num)) != 4:
        return 0
    
    num_list = sorted(str(num), reverse=True)
    for i in range(4, 0, -1):
        permuted_num = int(''.join(num_list[:i]))
        if is_prime(permuted_num):
            return permuted_num
        
    return 0

input_number = int(input())
result = largest_prime(input_number)
print(result)